The settling basins in water and wastewater treatment plants are typically circular or rectangular concrete structures that have sloped bottoms that collect and store sludge. The basins are periodically drained and the sludge that has accumulated is flushed out through drains in the basin floor. Removal of the accumulated sludge is an important part of the overall treatment process, particularly since anaerobic bacterial activity may develop in the collected sludge over time. Therefore, in an ideal situation, the sludge is drained or removed without disturbing the material through excavation or manual removal. Moreover, it is further desirable from a cost and efficiency standpoint to be able to effectively remove the sludge from the basin when accumulation requires its removal.
Methods of removing sludge on a more or less continuous basis have been developed in an effort to reduce the need to completely drain and flush the basin. Techniques such as header and lateral piping systems, scraper mechanisms and vacuum or suction removal systems have been tried, without complete success.
Sludge removal systems are known where a moving apparatus is directed across the floor of the basin to remove the sludge in its path. The unit can be guided by rails and may use static head or eduction to remove the sludge. In some variations, the sludge collection basin is cone-shaped and the sludge collection apparatus moves along a circular or spiral path over the surface of the cone. Such systems are cumbersome and expensive and require a complex system of moving parts and precision machinery prone to breakdown in the gritty environment of a sludge bed.
In other systems, the inherent problems of moving equipment are avoided by providing a series of fixed pipes to remove the sludge. The pipes are selected so that a number of perforated pipes of small diameter are connected to larger diameter pipes, which are in turn connected to a lesser number of larger diameter pipes, and so on, until ultimately, the piping "tree" meets at a single header pipe of relatively large diameter that is controlled by a single collection valve. The single valve is opened to flush sludge into drains by means of the static head of water in the basin. In order for fixed grid sludge collection systems to collect evenly, it is critical that the flow capacity of the grid be compatible with the flow capacity of the downstream piping including the header, valve and basin outlet. Therefore, it is necessary to either use a very large valve and basin outlet in combination with a grid covering the entire basin or to use multiple valves and basin outlets in combination with smaller grids.
Another limitation to fixed sludge collection systems is that sludge is often not efficiently removed because the water in the basin tends to flow around the sludge and into the collection system. Typically, sludge may flow into the collection system when the valve first opens, creating a hole or depression in the sludge. This depression is known as a "rat hole." Once this depression exposes the collection orifice, water enters the collection system rather than sludge. Fixed collection systems that have valves open for extended periods of time usually collect more water than sludge. The tendency toward "rat holing" is dependent upon certain characteristics of the sludge such as its composition, concentration, viscosity, and compressibility. Sludge found in water treatment processes may have different compositions, varying amounts of suspended solids, and therefore different characteristics. For example, alum sludge will have different characteristics than ferric sludge or lime sludge.
A limitation to systems using single large outlets is that the system must be operated to remove sludge from the entire basin as soon as one area exhibits significant sludge accumulation. This type of operation is inherently inefficient and either wastes water unnecessarily or allows sludge to accumulate more heavily than is optimal for the process. A limitation to systems using the multiple basin outlets is the expense of the outlets and the difficulty in retrofitting the basin.
Finally, sludge removal is also sometimes attempted by providing systems that float and skim the sludge from the water overlying the bottom of the basin. However, such systems are inefficient, expensive and require complex systems of piping and suction, the latter of which are prone to breakdown since a gritty slurry of sludge and water must be skimmed and pumped.
Additionally, all of the known systems discussed above are difficult if not impossible to retrofit into existing sludge collection basins.
Therefore, it would be desirable to provide a sludge collection system that is simple and reliable, while still effectively removing sludge. Additionally, it would be desirable to permit the amount of removal applied to various areas of the bed to be adjusted or varied based upon sludge accumulation patterns. Finally, such a system would ideally be adaptable for both new construction and retrofit applications.